Hydroxyl radical reaction rate constant model

Tropospheric photochemical air pollution has impacts on scales ranging from local to global. Reactions with hydroxyl radical (OH.) is the most important pathway of day time removal of organic pollutants in atmosphere. Theoretical approaches as QSAR/QSPRs are often used to predict the high risk of organic chemicals to reduce the time consuming, expensive and difficult experimental procedures. The crucial importance of the three central OECD principles for QSAR model validation is highlighted in a case study of tropospheric degradation of volatile organic pollutants (VOCs) by OH, applied to two CADASTER chemical classes (PBDEs and (benzo)triazoles). The application of any QSAR model to chemicals without experimental data largely depends on model reproducibility by the user. The reproducibility of an unambiguous algorithm (OECD Principle 2) is guaranteed by redeveloping MLR models based on updated version of DRAGON software for molecular descriptors calculation and some freely available online descriptors. The Genetic Algorithm has confirmed its ability to select similarly informative descriptors, independently on the input pool of variables. The ability of the GA-selected descriptors to predict chemicals, not used in model development, is verified by three different splittings (random by response, K-ANN and K-means clustering by structural similarity), thus ensuring the external predictivity of the new models (OECD Principle 4), independently of the training/prediction set composition, as verified by various validation parameters. The relevance of checking the structural applicability domain (OECD Principle 3) becomes evident on comparing the predictions for CADASTER chemicals, using the new models proposed herein, with those obtained by EPI Suite.